CS 320: Concepts of Programming Languages

Transcription

1 CS 320: Concepts of Programming Languages Wayne Snyder Computer Science Department Boston University Lecture 06: Useful Haskell Syntax, HO Programming Continued o Goodbye to Bare Bones Haskell: Built-in syntax for lists & tuples o Lambda expressions and Beta-Reduction o Let and Case Expressions Reading: Hutton Ch. 4 & 7 You should be starting to look through the Standard Prelude in Appendix B, particularly the list processing functions!

2 Useful Haskell Syntax: Built-In Types We have used Bare Bones Haskell notation for Lists, Pairs, and Triples in order to emphasize the importance of pattern-matching in defining functions. However, enough is enough! Here is a more convenient syntax which is built into the basic Haskell syntax (and not just implemented as functions in the Prelude): BB Haskell Flesh and Blood Haskell

3 Useful Haskell Syntax: Built-In Types We have used Bare Bones Haskell notation for Lists, Pairs, and Triples in order to emphasize the importance of pattern-matching in defining functions. However, enough is enough! Here is a more convenient syntax which is built into the basic Haskell syntax (and not just implemented as functions in the Prelude): BB Haskell Flesh and Blood Haskell Built in to the Prelude exactly as we presented it: Bool, True, False, &&,, not Built in types Integer, Double,... Main> Main> *

4 Useful Haskell Syntax: Built-In Tuples BB Haskell Main> P 3 True P 3 True Main> (P 4 (P True (-9))) P 4 (P True (-9)) Main> (T 3 5 9) T Main> (T 9 False 2) T 9 False 2 Main> fst (P 3 True) 3 Main> snd (P 3 (P True 2)) P True 2 Main> toleft (P 4 (P True (-9))) P (P 4 True) (-9) Main> p2t (P 4 (P True (-9))) T 4 True (-9)

5 Useful Haskell Syntax: Built-In Tuples BB Haskell Main> P 3 True P 3 True Main> (P 4 (P True (-9))) P 4 (P True (-9)) Main> (T 3 5 9) T Main> (T 9 False 2) T 9 False 2 Provided in Prelude Flesh and Blood Haskell Main> (3,True) (3,True) Main> (4,(True,(-9))) 4 (True,(-9)) Main> (3,5,9) (3,5,9) Main> (9,False,2) (9,False,2) Main> fst (3,True) 3 Main> snd (3,(True,2)) (True,2) Main> toleft (4,(True,(-9))) ((4,True),-9) Main> p2t (4,(True,(-9))) (4,True,-9) Main> (2,3,True,5, a,7,4, hi,5) (2,3,True,5, a,7,4, hi,5) Tuples can be any length, but fst and snd only work on pairs.

6 Useful Haskell Syntax: Built-In Lists BB Haskell Flesh and Bones Haskell Built in as part of syntax! Provided in Prelude Main>(Cons 3 (Cons 9 Nil)) Cons 3 (Cons 9 Nil) Main> head (Cons 3 (Cons 9 Nil)) 3 Main> tail (Cons 3 (Cons 9 Nil)) Cons 9 Nil Main> length (Cons 3 (Cons 9 Nil)) 2 Main> [] [] Main> 3:9:[] [3,9] Main> 3:[9] [3,9] Main> [3,9] [3,9] Main> head [3,9] 3 Main> tail [3,9] [9] Main> length [3,9] 2

7 Useful Haskell Syntax: Built-In Lists Start to become familiar with the list-processing functions in the Prelude, there are many useful functions already defined! See Hutton pp Main> [0,1,2] ++ [3,4] [0,1,2,3,4] Main> last [0,1,2,3,4] 4 Main> init [0,1,2,3,4] [0,1,2,3] Main> take 3 [0,1,2,3,4] [0,1,2] Main> drop 3 [0,1,2,3,4] [3,4] Main> takewhile (<3) [0,1,2,3,4] [0,1,2] Main> dropwhile (<3) [0,1,2,3,4] [3,4] Main> splitat 3 [0,1,2,3,4] ([0,1,2],[3,4]) Main> replicate 5 1 [1,1,1,1,1] Main> [0,1,2] ++ [3,4] [0,1,2,3,4] Main> reverse [0,1,2,3,4] [4,3,2,1,0] Main> map (^2) [0,1,2,3,4] [0,1,4,9,16] Main> filter even [0,1,2,3,4] [0,2,4] Main> concat [[0],[1,2],[3,4]] [0,1,2,3,4] Many more advanced functions can be found in Data.List.

8 Useful Haskell Syntax: Characters and Strings Characters (Hutton p.282) Main> 'a' 'a' Main> ['h','i','!'] "hi! Main Data.Char> islower 'a' True Main Data.Char> isupper 'a' False Main Data.Char> isalpha 'a' True Main Data.Char> isdigit 'a' False Main Data.Char> ord 'a' 97 Main Data.Char> chr 97 'a' Main Data.Char> digittoint '9' 9 Main Data.Char> inttodigit 4 '4' Main Data.Char> toupper 'a' 'A' Main Data.Char> tolower 'A' 'a' Main Data.Char> nextchar 'a' 'b'

9 Useful Haskell Syntax: Characters and Strings Strings are simply lists of Characters (Hutton p.282) Main> ['h','i','!'] "hi! Main> "hi " ++ "there" ++ "!" "hi there!" Main> "hi there"!! 3 't' Main> take 5 "hi there!" "hi th Main> words "hi there!" ["hi","there!"] Main> import Data.Char Main Data.Char> map toupper "hi there!" "HI THERE!" Any list function can be used on Strings. Check out Data.List! This nifty function is provided in the Prelude

10 Case Expressions A very useful kind of conditional expression is the case expression: case expression of pattern -> result pattern -> result pattern -> result... In other languages, the case statement is an alternative to a long nested ifthen-else, but in Haskell (of course!) it is more powerful, as it does pattern matching: describe :: [a] -> String describe [] = "empty" describe [x] = "singleton" describe _ = "big!" *Main> describe [4] "singleton" describe :: [a] -> String describe xs = case xs of [] -> "empty" [x] -> "singleton" _ -> big!"

11 Case Expressions This solves the problem that lambda expressions can pattern match, but not do multiple patterns: describe :: [a] -> String describe = \xs -> case xs of [] -> "empty" [x] -> "singleton" _ -> big!"

12 Beta Reduction and Let Expressions Recall: a lambda expression represents an anonymous function: makepair :: a -> b -> (a,b) makepair x y = (x,y) makepair x = \y -> (x,y) makepair = \x -> \y -> (x,y) Main> makepair 3 True (3,True) By referential transparency, we can simply use the lambda expression and apply it directly to arguments: Main> (\x -> \y -> (x,y)) 3 True (3,True)

13 Beta Reduction and Let Expressions We will study this much more in a few weeks, when we start to think about how to implement functional languages, but for now, we just define the concept of Beta-Reduction, which is simply substituting an argument for its parameter: ((\x -> <expression>) <argument>) => <expression> with x replaced by <argument> Examples: Main> (\x -> (x,x)) 4 (4,4) Main>(\x -> [3,x,9]) 4 [3,4,9] Main>(\x -> Just x) "hi" Just "hi" Main>(\x -> 5) 6 5 Main> (\x -> (\y -> (x,y))) 5 True (5,True) Main>(\x y -> [3,x,y]) 4 9 [3,4,9] Main>(\x y -> \z -> [x,y,z]) [2,4,9] Main> (\x -> (\x -> (x,x))) 5 True??

14 Beta Reduction and Let Expressions We will study this much more in a few weeks, when we start to think about how to implement functional languages, but for now, we just define the concept of Beta-Reduction, which is simply substituting an argument for its parameter: ((\x -> <expression>) <argument>) => <expression> with x replaced by <argument> Examples: Main> (\x -> (x,x)) 4 (4,4) Main>(\x -> [3,x,9]) 4 [3,4,9] Main>(\x -> Just x) "hi" Just "hi" Main>(\x -> 5) 6 5 Main> (\x -> (\y -> (x,y))) 5 True (5,True) Main>(\x y -> [3,x,y]) 4 9 [3,4,9] Main>(\x y -> \z -> [x,y,z]) [2,4,9] Main> (\x -> (\x -> (x,x))) 5 True (True,True) Why??

15 Scope in Haskell The scope of a variable (e.g., local variable, parameter) is the region of the program where it is legal to refer to that variable. Main> x <interactive>:14:1: error: Variable not in scope: x Main> Main> x = 4 Main> x 4 In Java there are several kinds of scoping rules...

16 Digression: Scope in Java The scope of a variable (e.g., local variable, parameter) is the region of the program where it is legal to refer to that variable. Main> x <interactive>:14:1: error: Variable not in scope: x Main> Main> x = 4 Main> x 4 In Java there are several kinds of scoping rules...

17 Digression: Scope in Java The scope of a variable (e.g., local variable, parameter) is the region of the program where it is legal to refer to that variable. Main> x <interactive>:14:1: error: Variable not in scope: x Main> Main> x = 4 Main> x 4 In Java there are several kinds of scoping rules...

18 Scope in Let Expressions The scope of a lambda parameter is the expression to the right of the -> (\x -> <expression>) Scope of x To find the parameter associated with an instance of a variable in the expression, look for the closest enclosing binding of the variable: (\x -> \ys -> (length (take x ys)))

19 Scope in Let Expressions: Hole in Scope To find the parameter associated with an instance of a variable in the expression, look for the closest enclosing binding of the variable: (\x -> \ys -> (length (take x ys))) Some weird things can happen when there is more than one occurrence of the same variable: Main> (\x -> ((\x -> (take x [1,2,3,4,5]) ) 3) ++ x) [7] [1,2,3,7] Main>(\x -> (\x -> (x,x))) 5 True (True,True) Hole in scope of outer x

20 Digression: Scope in Java Java allows multiple declarations of the same variable if one is a field and one is a local variable (either a parameter or a local variable), creating a hole in the scope of the field declaration:

21 Digression: Scope in Java But Java does NOT allow multiple declarations (and hence avoids the hole in scope issue) for two local variables:

22 Digression: Scope in Java But Java does NOT allow multiple declarations (and hence avoids the hole in scope issue) for two local variables:

23 Digression: Scope in C C allows multiple declarations without many restrictions:

24 Let Expressions in Haskell In Haskell we create local variables using let: (let x = <expr1> in <expr2>) cylinder r h = let sidearea = 2 * pi * r * h toparea = pi * r ^2 in sidearea + 2 * toparea Equivalent to a lambda application: ((\x -> <expr2>) <expr1>) Except that you can have multiple bindings in the same let. Scope of local variables let sq x = x * x in (sq 5, sq 3, sq 2) => (25,9,4) => 65 let x = 5 in let y = 2 * x in let z = x + y in (\w -> x * y + z) 10

25 Let Expressions in Haskell Haskell let s you define local variables any time you want with let (and where), and therefore hole in scope issues become relevant. Notice the enormous flexibility of Haskell and the referential transparency principle: You can use these kinds of expressions nearly anywhere! (let sq = (\x -> x*x) in \x -> (x,sq x) ) 5 => (5,25) (\x -> case x of 1 -> \x -> x > \x -> x * 2 _ -> \x -> x ) 2 6 => 12